If your tweet contains a link that Twitter has blocked, your post will fail and you'll see the following error:
We can't complete this request because this link has been identified by Twitter or our partners as being potentially harmful.
In this case, you will need to edit the post and remove the link in order to publish it.
You can read more about Twitter's approach to blocking links here, which covers the categories of links that they're likely to block (e.g. malicious links that could steal personal information, spammy links that mislead people or disrupt their experience, hateful conduct, etc). If you believe the link has been blocked by Twitter in error, you can report it to them via this form.